Heart failure (HF), the final manifestation of most cardiovascular pathologies, is a devastating disease with poor prognosis. Pathologic cardiac remodeling (hypertrophy, fibrosis) is a known predictor of clinical outcome. We and others have demonstrated that pathologic activation of the cardiac fibroblast (CF) (support cell) after cardiac injury releases various pro-hypertrophic/inflammatory mediators that target both cardiomyocytes (CM) (functional cell) and fibroblasts to exacerbate remodeling. Elucidating mechanisms of this pathologic communication may hold therapeutic promise. Mixed Lineage Kinases (MLKs) are a family of stress-activated dual-specificity MAPKKKs upstream of JNK and p38 MAPKs. MLK3 in particular has been directly implicated in HIV-associated neurodegeneration (HAND), where it mediates microglial (support cell) activation neuronal (functional cell) damage. The similar pathogenic mechanisms of HAND and HF, namely deleterious support cell-functional cell communication, suggest a similar pathologic role for MLK3 in HF. However, the functional role(s) of MLKs in the heart remain largely unknown. We have generated global MLK3-/- and CF-targeted conditional MLK3fl/fl mice, in which we have found no basal cardiac phenotype; however, they are protected following myocardial injury. We have synthesized MLK3-specific inhibitory compounds, including URMC-099 (CNS-penetrant) and URMC-128 (CNS-impenetrant). URMC-099 reduced pathologic CF activation in vitro; it significantly attenuated cardiac hypertrophy and reduced interstitial fibrosis in pharmacologic and surgical models of HF, with no additive benefit in MLK3 global or conditional null mice. Our hypothesis is that MLK3 plays an important role in pathologic CF activation and HF progression and that its inhibition holds therapeutic promise for HF. To address our hypothesis, we propose the following Aims: Aim 1: Determine the therapeutic efficacy and specificity of MLK3 inhibition or ablation in clinically-relevant HF models in wild type, MLK3-/- and inducible CM or CF MLK3-/- mice. This aim will investigate the therapeutic efficacy and specificity of MLK3 pharmacologic inhibition (using our structurally distinct CNS-penetrant or -impenetrant compounds) and/or ablation (global, inducible CM, inducible resident or activated CF-restricted null mice) in ischemia-reperfusion (I/R) HF. Aim 2: Elucidate the cellular mechanisms underlying MLK3 inhibition and provide validation of MLK3 as a therapeutic target in both mouse and human HF. This aim will test the hypothesis that MLK3 inhibition or ablation will attenuate pathologic CF activation and subsequent CF/MF-CM pathologic crosstalk, using primary rodent CF and CM (Aim 1) subjected to pathogenic insults. Importantly, similar assays will ensue in human HF cardiac cells. MLK3 gain of function studies in vitro and in vivo will complement these studies. Our results may elucidate a new approach for treating HF and a novel paradigm of pathologic support cell-functional cell cross talk in the pathophysiology of multiple disease states.